Process for physico-chemical treatment of water and installation for embodiment of this process

ABSTRACT

The invention concerns a method for the physico-chemical treatment of water characterized in that it comprises at least a step consisting in causing the water to be treated in at least one settler providing non-thickened sludge and in at least one settler-thickener treating the sludge from the two settlers and part of the water to be treated. The invention also concerns a plant for implementing said method comprising means for supplying the water to be treated ( 6 ), at least a settler providing non-thickened sludge ( 2 ), at least a settler-thickener ( 5, 5   a ), means for conveying ( 7 ) at least part of the non-thickened sludge coming from the settler providing non-thickened sludge ( 2 ) to the settler-thickener ( 5, 5   a ), means for evacuating ( 9, 9   a ) treated water coming from said settler providing non-thickened sludge ( 2 ) and from said settler-thickener ( 5, 5   a ) and means for evacuating ( 8 ) the thickened sludge coming from said settler-thickener.

BACKGROUND OF THE INVENTION

This invention relates to the field of water treatment for purificationor for making it drinkable, and particularly but not exclusively urbanor industrial waste water and rain water.

Standard practice includes various physicochemical water treatmenttechniques making use of different types of settling tanks that canachieve different hydraulic treatment velocities that dependparticularly on their architecture and their size.

With simple settling tanks it is hardly possible to achieve velocitieshigher than 2.5 m/h, but these velocities can be as high as 10 to 15 m/hwhen the internal structure of the settling tanks used includes lamellarelements. In particular, lamellar settling tanks of this type aremarketed by the Applicant under the trade name MULTIFLO (registeredtrademark). Furthermore, when the settling step itself is preceded by acoagulation-flocculation step, velocities of the order of 30 m/h, oreven 80 m/h, can be achieved if a sludge recirculation step is included.An even more advanced technique developed by the Applicant and marketedunder the brand ACTIFLO (registered trademark) consists of usingsettling tanks with flocs ballasted by appropriate ballasting material,and particularly micro-sand. A technique of this type is described indetail in patents FR 2 627 704 and FR 2 719 734 issued to the Applicant.With this type of ballasting of flocs formed in settling tanks,treatment velocities of 50 m/h and up to 200 m/h can be achieved.

Settling tanks of the type described above produce unthickened sludge.The sludge concentration at the outlet from ballasted floc settlingtanks using a granular material usually varies between 0.5 g/l and 20g/l of dry matter (DM). Therefore, this sludge has to be thickened usingthickeners to achieve a concentration of between 20 and 250 g/l of DM(corresponding to a dryness ratio of 2 to 25%), and preferably between20 and 90 g/l of DM (corresponding to a dryness ratio of 3 to 9%).

Within the framework of the ACTIFLO technique described above, athickener is designed to recover sludge from the settling tank, andoverflow water from thickeners is returned to the top of the settlingtank. This return of water from the thickener overflow to the beginningof the line increases the water influent entering the installation to betreated.

In other settling tanks according to the standard practice, the sludgethickening function is built into the settling tanks themselves. Thesesettling tanks are then conventionally called “thickener settlingtanks”. In this respect, note the technique also developed by theapplicant and marketed under the name MULTIFLO E or EF, or many othertechniques described in patent document FR 2 156 277. Similar techniquesalso use sludge recirculation (U.S. Pat. No. 4, 388, 195, FR 2 553 082).With these thickener settling tanks, hydraulic treatment velocities ofthe order of 30 to 50 m/h, or even 80 m/h in some cases, can be obtainedto produce a sludge with a dryness that can vary from 2 to 25%, with anaverage of 3 to 8% (corresponding to 30 to 80 g/l of dry matter).However, they cannot obtain velocities as high as the velocitiespossible with ballasted floc settling tanks.

Concerning the technique used for thickeners, static thickeners based ona load on the base mat corresponding to the maximum allowable mass flowof dry matter per unit area. Conventionally, this base mat load isbetween 20 and 100 kg/DM/m²/d. A lamellar thickener preceded by anunthickened sludge flocculation step can increase base mat loads tobetween 500 and 3000 kg DM/m²/d, and sometimes more.

SUMMARY OF THE INVENTION

The main purpose of this invention is to describe a process capable ofbenefiting from the advantages of ballasted floc settling tanks with agranular material that can operate at very high hydraulic velocities (upto 200 m/h as mentioned above) and the advantages of lamellar thickenersettling tanks that give very good thickening of the sludge, but themaximum hydraulic velocities are not as high as the velocities possiblewith ballasted floc settling tanks.

Another purpose of this invention is to describe such a process makinguse of settling tanks with base mat areas smaller than the areas thatwould conventionally be used according to standard practice foroptimized hydraulic treatment velocities and sludge drynesses, andtherefore reduce the size of the corresponding installations forembodiment of this process.

Another objective of this invention is to divulge such a process thatcan overcome the need to return water overflowing from thickeners to thebeginning of the installation.

Another objective of this invention is to propose a process that canenable efficient treatment of water when the flow or mass content of thewater to be treated increases, even suddenly, for example duringrainstorms.

These various objectives are achieved according to the invention whichrelates to a process for the physicochemical treatment of water,characterized in that it comprises at least one step that consists ofpassing the water to be treated in at least one settling tank of thetype that produces unthickened sludge, and in at least one thickenersettling tank, the said thickener settling tank treating the sludge fromthe two settling tanks and part of the water to be treated.

Therefore the main innovation of this process according to thisinvention is the characteristic according to which the thickenersettling tank is not used only to treat sludge from a settling tankproducing thickened sludge, but it is also used to treat some of thewater itself (influent). As described in detail later, it is thuspossible to reduce the size of the installations since the settling tankfunction of the thickener settling tank can increase the availablesettling area to treat the influent.

According to one particularly interesting variant of the invention, thesaid settling tank of the type producing unthickened sludge is aballasted floc settling tank ballasted by at least one granularmaterial, preferably micro-sand, and the said thickener settling tank isa lamellar thickener settling tank. The granular material willpreferably consist of micro-sand, although almost any granular materialknown to those skilled in the art could be envisaged.

The invention thus combines the advantages of settling tanks with flocsballasted by a granular material capable of functioning at very highhydraulic velocities (up to 200 m/h as mentioned above), and theadvantages of lamellar thickener settling tanks that provide a very goodsludge thickening function. Thus, in a preferred variant of theinvention, ACTIFLO ballasted floc settling tanks can be used withMULTIFLO E or EF thickener settling tanks marketed by the Applicant anddescribed above. Within the framework of this type of preferredembodiment, it will be noted that the sludge originating from such asettling tank with flocs ballasted by a granular material thickens verywell and does not cause any disturbance in the thickener settling tank.All that is necessary to achieve this is to reduce the water flowtreated on this structure, and increase the water flow treated on thesettling tank with flocs ballasted by the granular material.

The size of a thickener settling tank depends on two parameters, namelythe hydraulic velocity for settling and the mass load on the base matfor thickening. The hydraulic velocity is usually the controllingfactor. With the process according to the invention, returningunthickened sludge from the lamellar settling tank with flocs ballastedby the granular material, the concentration of sludge in the thickenersettling tank increases such that the maximum allowable hydraulicvelocity can be increased, and thus the maximum hydraulic velocitycorresponding to the velocity of the mass load on the base mat can beused. Therefore, with the process according to the invention, thethickener settling tank is sized as a function of the maximum allowableload on the base mat and the ballasted floc settling tank is sized tohandle the additional amount that cannot be accepted hydraulically bythe thickener settling tank.

According to another embodiment of the invention, the lamellar thickenersettling tank used may also include sludge recirculation. In this case,the process according to the invention advantageously comprises a stepin which the said unthickened sludge from the settling tank of the typeproducing unthickened sludge is mixed with sludge recirculated from thelamellar thickener settling tank with sludge recirculation.

According to one variant of the invention, the process consists ofpassing water to be treated through the said settling tank producingunthickened sludge and then in the said thickener settling tank, thesaid settling tanks then being supplied in series.

According to another variant, the said settling tank producingunthickened sludge and the said thickener settling tank can beconfigured in parallel.

This type of variants to the process will be used to adapt it to theconditions in which it is used and particularly to variations in thematerial and water flows to be treated.

According to another interesting aspect of the invention, the processcomprises an additional step consisting of adding at least one reagentin order to improve settling and thickening of the sludge in the saidthickener settling tank. This reagent may be added continuously to thewater, in other words the said reagent is added directly into theinfluent to be treated or in a coagulation-flocculation step included onthe inlet side of the said thickener settling tank, or included in it.Advantageously, it may be envisaged as part of an installation using theECOFLOC prolonged coagulation process developed by the applicant anddescribed in French patent FR 2 631 021.

The main advantages of this invention that consists of the combined useof a settling tank of the type producing unthickened sludge (preferablya settling tank with flocs ballasted by at least one granular material)and a thickener settling tank (preferably lamellar with or withoutsludge recirculation) are as follows.

The process according to the invention is more competitive thanprocesses according to standard practice mentioned in the preamble(settling tank + thickener or settling tank with integrated thickener)particularly due to the fact that it reduces the dimensions ofinstallations necessary for its embodiment.

This invention eliminates the need to return overflows from thickenersto the beginning of the installation, which are necessary when settlingtanks coupled with a sludge thickener are used alone.

Furthermore, if unthickened but pre-flocculated sludge obtained from theballasted floc settling tank is injected into the thickener settlingtank, it is possible to:

obtain a higher concentration of SS (suspended solids) at the inlet tothe thickener settling tank, that enables optimized hydraulic treatmentvelocities within this thickener settling tank;

obtain sludge with a higher dryness ratio;

if applicable, reduce the recirculated sludge flow rate when it isdecided to use a thickener settling tank with sludge recirculation;

possibly reduce the content of reagents used in the thickener settlingtank;

increase the load on the thickener settling tank base mat (since thesludge is mixed, it thickens more quickly so that the load on the basemat of the thickener part of the thickener settling tank can beincreased and therefore improved) while keeping the water qualityconform with the required quality;

easily accommodate variations in pollutant flows to be treated;

adapt the process to treat dry weather or wet weather water and materialflows.

The invention also relates to any installation for use of the process asdescribed above, characterized in that it comprises means of conveyingthe water to be treated to the inlet, at least one settling tank of thetype producing unthickened sludge, at least one thickener settling tank,means of transporting at least part of the unthickened sludge from thesettling tank of the type producing unthickened sludge to the thickenersettling tank, means of conveying treated water away from the saidsettling tank of the type producing unthickened sludge and from the saidthickener settling tank and means of conveying thickened sludge awayfrom the said thickener settling tank.

If required, this type of installation could be preceded by conventionalmeans for preliminary treatment of the water to be treated (for examplescreening, sand removal and degreasing, or simply screening). Sludgefrom thickened sludge removal means can be conveyed in a known manner tomeans of treatment of this sludge. Finally, a post-treatment can then beapplied to treated water outlet from the installation.

Preferably, said means of conveyance of water to be treated of theinstallation according to the invention are such that the said settlingtank of the type producing unthickened sludge and the said thickenersettling tank can be configured either in series or in parallel. Thistype of installation could thus be easily configured to adapt tovariations in content and/or flow of the influent. Advantage could alsobe taken of this characteristic when the installation is started up.

Preferably, the said settling tank of the type producing unthickenedsludge is a lamellar settling tank with flocs ballasted by at least onegranular material (for example such as an ACTIFLO settling tank marketedby the Applicant).

Also preferably, the said thickener settling tank is a lamellarthickener settling tank or a lamellar thickener settling tank withsludge recirculation.

Advantageously, the installation also comprises means of adding at leastone coagulating or flocculating reagent in the said thickener settlingtank.

The invention also relates to processes for use of this installation.

In particular, the invention also relates to a process that consists ofsupplying the said settling tank of the type producing unthickenedsludge and the said thickener settling tank in series when starting upof the installation, and supplying the said settling tanks in parallelonce the installation has been started up.

The invention also relates to a process that consists of supplying thesaid settling tank of the type producing unthickened sludge and the saidthickener settling tank in series when a normal flow and/or inlet ofwater to be treated is input into the said installation, and supplyingthe said settling tanks in parallel during peak water or material flows.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its many advantages, will be more easily understoodafter reading the following description of several non-restrictiveembodiments of the invention with reference to the drawings, in which:

FIGS. 1 and 2 diagrammatically show different installations usingphysicochemical treatment processes according to standard practice;

FIG. 3 shows a first embodiment of an installation making use of theprocess according to the invention;

FIGS. 4 to 8 show several operating configurations of a secondembodiment of an installation using the process according to theinvention and;

FIGS. 9 and 10 show several operating configurations of a thirdembodiment of an installation using the process according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a first installation for physicochemical treatment of wateraccording to a first known technique. This installation includes apreliminary treatment unit 1 (possibly simply a screen), a settling tankof the type producing unthickened sludge 2 (for example a MULTIFLOlamellar settling tank or an ACTIFLO ballasted floc settling tank), fromwhich unthickened sludge and treated water are extracted. Theunthickened sludge is conveyed towards a thickener 3 to increase itsdryness ratio. Thickened sludge is then transferred to a sludgetreatment unit 4. The overflow water from the thickener 3 is returned tothe inlet of the settling tank 2. The type of settling tank described inthis known technique can produce high treatment velocities, but requiresthe use of an associated thickener and therefore the use of largeinstallations.

FIG. 2 shows a second installation for physicochemical treatment ofwater according to a second known technique. This installation includesa preliminary treatment unit 1 (possibly simply a screen), and alamellar thickener settling tank 5 (for example a MULTIFLO E or EFlamellar thickener settling tank) at the outlet from which thickenedsludge and treated water are obtained directly, the thickened sludgethen being conveyed to a sludge treatment unit 4. (Note that thelamellar thickener settling tank can also be composed of a lamellarsettling tank with sludge recirculation).

Thickener settling tanks according to this second known technique canproduce sludge with a high dryness ratio, but are incapable of achievingtreatment velocities possible with first known technique.

FIG. 3 shows an installation making use of the process according to theinvention.

This installation comprises preliminary treatment means 1, a settlingtank of the type producing unthickened sludge composed of a ballastedfloc settling tank 2, a lamellar thickener settling tank 5, conveyancemeans 6 bringing the water to be treated into the ballasted flocsettling tank 2 and into the lamellar thickener settling tank 5, means 7of conveying unthickened sludge from the ballasted floc settling tank 2to the thickener settling tank 5, means of evacuation of treated water 9from the two settling tanks 2, 5 and means 8 of evacuating the thickenedsludge from the thickener settling tank 5 to a sludge treatment unit 4.The installation also includes means 10 of adding at least one reagentinto the thickener settling tank.

The following is a comparative example of the sizes of an installationbased on known techniques of the type shown in FIG. 2, and aninstallation according to the invention of the type shown in FIG. 3.

This comparative example is based on the following basic data:

Maximum daily mass flow to be treated in the installation: 120000kg/SS/d

Maximum flow of water to be treated: 10000 m³/h Maximum allowable basemat load in the thickener: 2000 kg/m²/d.

For an installation according to prior art of the type shown in FIG. 2with a maximum treatment velocity (on the base mat) of 40 m/h, therequired area on the base mat is the largest of the areas calculatedbelow:

10 000 m³/h/40 m/h=250 m²

For an installation according to the invention of the type shown in FIG.3 using a ballasted floc settling tank allowing a maximum velocity onthe base mat of 80 m/h, the required area is equal to the sum of thearea of the thickener settling tank base mat and the area of theballasted floc settling tank base mat.

The area of the base mat of the thickener settling tank is sized as afunction of the mass flow, namely:

120000 kg/d/SS/2000 kg/m²/d=60 m²

Given the maximum allowable velocity on the base mat (40 m/h) of thethickener settling tank, the maximum allowable flow on the structure canbe deduced as follows:

60 m² *40 m/h=2400 m³/h

The additional flow to be treated on the ballasted floc settling tank iscalculated as follows, and will be used to calculate the required areaof its base mat: 10 000 m³/h−2 400 m³/h+(hourly flow of unthickenedsludge returning into the thickener settling tank (about 5%))=8 000m³/h.

Knowing the maximum velocity at the base mat of the ballasted flocsettling tank (80 m/h), the required area at the base mat is deduced asfollows:

8000 m³/h/80 m/h=100 m²

Therefore within the framework of the invention, the total area of thebase mat is 60 m²+100 m²=160 m², compared with 250 m² for theinstallation according to known techniques, giving a saving of 90 m²(36%).

A second embodiment of an installation according to the invention isshown with reference to FIGS. 4 and 5. This installation comprisespreliminary treatment means 1, a ballasted floc settling tank 2, alamellar thickener settling tank 5 a provided with means ofrecirculating thickened sludge 5 b, means 6 a, 6 b of conveying water tobe treated into the ballasted floc settling tank 2 and into the lamellarthickener settling tank 5 a, means 7 of conveying unthickened sludgefrom the ballasted floc settling tank 2 into the thickener settling tank5 a, means of evacuation of treated water 9, 9 a from the two settlingtanks 2, 5 a, and means of evacuation 8 of thickened sludge from thethickener settling tank 5 b to a sludge treatment unit 4. Theinstallation also comprises means 10 of adding at least one reagent intothe thickener settling tank 5 b.

This installation can be used in dry weather with rain water flow peaks3 to 4 times greater than flow peaks in dry weather, by passing theentire incoming flow into the thickener settling tank (see configurationin FIG. 4).

In wet weather (see configuration in FIG. 5), the thickener settlingtank will be used to thicken unthickened sludge from the ballasted flocsettling tank 2. The thickener settling tank will be designed to accepta small proportion of the flow to be treated and thus contributes toreducing the dimensions of the ballasted floc settling tank. This typeof operation also has the advantage that it can reduce or even eliminatethe addition of reagents necessary for operation in dry weather, whentreatment requirements are low.

As mentioned above, the settling tank of the type producing unthickenedsludge (corresponding to a settling tank 2 with flocs ballasted by agranular material, within the framework of this embodiment) and thethickener settling tank (corresponding to a lamellar thickener settlingtank with sludge recirculation 5 a within the framework of thisembodiment) can be supplied in series or in parallel. The inletconveyance means 6, 6 a and the treated water outlet conveyance means 9,9 a can be varied depending on the configuration.

In configuration shown in FIG. 6, these two settling tanks are suppliedin series. Very good quality water can be obtained at all times withthis configuration. With this configuration, unsettled sludge can bereturned to the thickened sludge recirculation 5 b used to create andmaintain the sludge bed in the thickener settling tank 5 a. This cangive a better consistency of the sludge bed, so that it is possible towork at greater hydraulic velocities while maintaining a high qualitywater at the outlet.

If the flow or the material contained in the water to be treatedincreases, the two settling tanks 2, 5 a may be supplied in parallelusing the configuration shown in FIG. 7. When the hydraulic or mass flowpeak has finished, the two settling tanks 2, 5 a may once again besupplied in parallel.

The configuration in series may also be used when the installation isstarted in order to facilitate creation of the sludge bed in thethickener settling tank 5 a. Any expert in the subject knows that sometime is necessary to create this sludge bed, however the ballasted flocsettling tank 2 starts up instantaneously even if the structure isempty. The unthickened ballasted sludge that is initially sent into thethickened sludge recirculation loop 5 b accelerates creation of thesludge bed. Once the sludge bed has been formed, the two settling tanksmay be supplied in parallel.

Another startup possibility shown in FIG. 8 consists of starting theballasted floc settling tank 2 and evacuating the treated water fromthis settling tank. During this time, unthickened sludge is transferredto the lamellar thickener settling tank 5 a that is used only for itsthickening function. As long as the water outlet from the thickenersettling tank 5 a is not conform, the water is returned to the inlet tothe station to be treated in the ballasted floc settling tank 2. As soonas the thickener settling tank 5 a is working correctly, as a settlingtank and as a thickener, the treated water is released directly into thenatural environment or the next treatment, and operation in parallel canbe started as shown in FIG. 7.

The installation shown in FIGS. 9 and 10 is not different from theinstallation shown in FIGS. 4 to 8, except that the lamellar thickenersettling tank 5 a is located on the inlet side of the ballasted flocsettling tank 2.

FIG. 9 shows how the lamellar thickener settling tank 5 a is put intoseries with the lamellar ballasted floc settling tank 2 following it, ifa constant quality is to be obtained regardless of the water flow to betreated. In this case, the thickener settling tank 5 a will operate athydraulic velocities greater than the velocities normally required (inpractice greater than 50 m/h), the water at the outlet from thethickener settling tank 5 a will be degraded without causing anydisturbance in the ballasted floc settling tank 2. This settling tank isused for a refining treatment to eliminate residual suspended solids(SS) and parasites, particularly such as helminth eggs. During peakwater or material flows, the two settling tanks are supplied in parallelusing the configuration shown in FIG. 10.

The embodiments of the invention described are not intended to reducethe scope of the invention. Many modifications can be made to themwithout going outside the framework of this invention.

What is claimed is:
 1. A physiochemical treatment process for treatingwater, comprising: directing water to be treated through a firstsettling tank that produces a relatively unthick sludge; directing thewater to be treated through a second settling tank that produces arelatively thick sludge; and directing the relatively unthick sludgefrom the first settling tank through the second settling tank.
 2. Themethod of claim 1 including splitting the water to be treated into twostreams and directing one stream through the first settling tank thatproduces the relatively unthick sludge and directing a second stream ofthe water to be treated through the second settling tank that producesthe relatively thick sludge.
 3. The method of claim 2 wherein the firstand second settling tanks are disposed in parallel relationship.
 4. Themethod of claim 2 wherein the relatively thick sludge produced by thesecond settling tank is made up in part at least of sludge resultingfrom the second stream of water directed through the second settlingtank and the relative unthick sludge transferred from the first settlingtank to the second settling tank.
 5. The method of claim 4 includingdirecting the relative thick sludge produced by the second settling tankto a sludge treatment area.
 6. The method of claim 4 wherein each of thefirst and second settling tanks produce a treated effluent water stream.7. The method of claim 1 including adding a reagent to the secondsettling tank.
 8. The method of claim 7 wherein the reagent added to thesecond settling tank includes a coagulant—flocculant.
 9. The method ofclaim 1 including subjecting the wastewater directed to the firstsettling tank to ballasted flocculation.
 10. The method of claim 9wherein the second settling tank comprises a lamellar thickener settlingtank.
 11. The method of claim 1 wherein the first settling tankgenerally produces sludge having 20 g/l or less of dry matter, and thesecond settling tank generally produces sludge having a dry matter ofgreater than 20 g/l.
 12. The method of claim 1 wherein the dry matter ofthe sludge produced by the first settling tank ranges generally betweenapproximately 0.5 g/l and 20 g/l, and the sludge produced by the secondsettling tank produces sludge having dry matter of approximately 20 g/lto 250 g/l.
 13. The method of claim 1 wherein the first and secondsettling tanks are arranged in series.
 14. The method of claim 13wherein substantially all of the water treated by the two settling tankspasses through at least one of the settling tanks.
 15. The method ofclaim 1 including recycling at least a portion of the relatively thicksludge back through the second settling tank.
 16. The method of claim 1wherein the first and second settling tanks are disposed in series suchthat the water being treated is directed through one settling tankbefore being directed through the other.
 17. The method of claim 16including directing the water to be treated through the first settlingtank and then directing the resulting relatively unthick sludge andaccompanying water to be treated from the first settling tank to thesecond settling tank.
 18. The method of claim 16 including directing thewater through the second settling tank prior to the water being directedthrough the first settling tank.
 19. The method of claim 1 wherein inrelatively dry weather the method includes bypassing the first settlingtank and directing the water to be treated to the second settling tank.20. The method of claim 1 wherein each of the first and second settlingtanks separate sludge from water so as to produce a treated watereffluent.
 21. An installation for the physiochemical treatment of water,comprising: a first settling tank for receiving water to be treated andfor producing a relatively unthick sludge; a second settling tank forreceiving water to be treated and for producing a relatively thicksludge; at least one line for directing water to be treated to both saidfirst and second settling tanks; and a second line interconnectedbetween the first settling tank and the second settling tank fortransferring at least a portion of the relatively unthick sludge fromthe first settling tank to the second settling tank.
 22. Theinstallation of claim 21 including a recycle line associated with thesecond settling tank for recycling the relatively thick sludge throughthe second settling tank.
 23. The installation of claim 21 wherein thefirst and second settling tanks are disposed in parallel relationshipand wherein at least one line for transferring water to be treated toboth said first and second settling tanks includes a pair of lines witheach line being directed to one of the settling tanks.
 24. Theinstallation of claim 21 wherein the first settling tank comprises aballasted floc settling tank.
 25. The installation of claim 24 includinga sludge treatment area and a third line interconnected between thesecond settling tank and the sludge treatment area for transferringrelatively thick sludge from the second settling tank to the sludgetreatment area.
 26. The installation of claim 21 wherein the secondsettling tank comprises a lamellar thickener settling tank.
 27. Theinstallation of claim 21 wherein the first and second settling tanks aredisposed in series and wherein at least one line for directing water tobe treated to the first and second settling tanks includes one linedirected to one of the settling tanks and another line directed from theone settling tank to the other settling tank.
 28. The installation ofclaim 21 wherein the first and second settling tanks are disposed inseries and wherein the first settling tank is disposed on the inlet sideof the second settling tank.
 29. The installation of claim 21 whereinthe first settling tank is adapted to produce sludge having a dry matterconcentration of approximately 20 g/l or less, and wherein the secondsettling tank is adapted to produce sludge having a dry matterconcentration of approximately 20 g/l or greater.